Ah, that may change things a bit.

The CR-718 uses a pre-amp, like the original CR-814 did. If the pre-amp is still present, and it may well be, then you can carry on, just a matter of identifying all the wiring and having connectors on hand that fit the back of the radio. It may make for a fair bit of splicing, but it is doable.

Odds are your Pioneer aftermarket radio doesn't use a pre-amp unless the buyer was specifically looking for one to match the original or wanted to drive larger speakers. But odds are the original pre-amp is still there whether or not your Pioneer uses a pre-amp.

If the pre-amp does happen to have been removed, well you could always go searching for a used one, but honestly is it really worth all the effort?

Apart from being able to support an original optional CD player (as seemingly existed according to one source I read), there's nothing magical about the CR-718s other than it preserves the authenticity of the car for classic car looks and designation and it supports cassettes, which few current aftermarket radios do as CD players are much more popular these days. Even then, CDs players are now also pretty much passe what with the adevent of USB, MP3 aand bluetooth connectivity. You pretty much need to go with a double height radio if you want more function buttons and a player in addition to all the modern features, connectors and larger displays. With effort I'm sure you could put a double height or 1-1/2 height radio in a 740/940, but it would require a fair bit of customization.

The CR-718s are also just as prone, as many older radios are, to the volume rheostat wearing out which manifests itself as crackle through the speakers, dead/intermittent speakers, difficulty in fine tuning volume levels, balance or even getting enough volume to listen. If I really hated the Pioneer, then I'd think of looking for a different aftermarket radio that suits my tastes and has modern features (start by checking Crutchfield.com). I'll be honest with you, the newer radios (as in this century) tend to be a single push knob with nested menus that are nowhere near as simple to operate as the older radios, but the sound quality is generally just as good or better and of course you get a lot more features, many of them rather useful. You will eventually get used to a newer style radio, just don't accidentally drive over any cliffs in the process of trying to read the manual, looking at the button labels or trying to decipher tiny icons.

Just a note with non-Volvo radios. If you've got a power antenna, as most sedans do, then you need to ascertain whether the radio provides a power antenna signal. If not, then you need to determine which wires to connect to in order to maintain antenna functionality, even if it's just to splice into the power feed from the ignition switch and have it go up every time you turn the key on. Separate dash toggle switches are often used for over-riding antenna operation, such as the later 740s. Additionally, if you've got a 940 wagon with the FM antenna in the left rear quarter window glass then you have to connect the antenna amplifier located at the rear to power. Do it exactly as you would the power antenna, the wiring in the vehicle is the same other than the connections at the rear.

Another option if the pre-amp is no longer present or working and if you really hate the Pioneer, is to go looking for another single height slot genuine Volvo radio that fits the 700/900 series and doesn't use a pre-amp. The CR-915 is an example, I imagine there may be others. Used Volvo radios can often be had from yards and on eBay for like $25-$45, but again, make sure to get the rear connectors.

Radio lock codes can be obtained from Volvo through your Volvo dealer using the radio serial number (on top of the radio) and/or VIN (request both to be safe). You can now also do it yourself online, but only using the VIN, which of course is only good for the factory installed radio

A quick search did not find the wiring connections for the CR-718, but I'm sure with a bit of effort it's out there somewhere, either in a copy of the green Volvo electrical and wiring diagram for your year or elswhere in a blog or Volvo user site.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now